Our long-term goal is an identification and understanding of those molecular mechanisms which lead to improved myocardial postischemic function. The unifying hypothesis of the present proposal is specific PKC-dependent receptor agonists protect myocardium from ischemic dysfunction through a series of second messengers involving PKC, phosphatases, and MAPK. Activation of this pathway changes the phosphorylation state of cardiac myofilament proteins which leads to a decrease in Ca2+ -dependent actomyosin ATPase. Conservation of ATP by actin and myosin improves postischemic function by allowing continued activity of critical ATP-dependent pumps and ion channels during ischemia. Our sub-hypotheses / aims are: Hypothesis 1: Moderate decreases in myocardial Ca2+-dependent actomyosin ATPase, independent of second messenger activation, will increase postischemic functional recovery. Hypothesis 2: Phosphatases are an integral part of the second messenger pathway leading to protection of the heart from ischemic damage. Cardioprotective agents activate PKC which alters phosphatase activity. Hypothesis 3: Phosphatases alter cardiac myofilament phosphorylation state which in turn decreases actomyosin ATPase, conserves ATP, and protects the heart from postischemic dysfunction. Hypothesis 4: MAPKs are an integral part of the second messenger pathway leading to protection of the heart from ischemic damage. Cardioprotective agents activate PKC which in turn alters MAPK activity. Hypothesis 5: Activation of MAPK-phosphatase pathway leads to alter myofilament phosphorylation state which decreases actomyosin ATPase, conserves ATP, and protects the heart from postischemic dysfunction. Methods to be used include (i) determination of postischemic pressures and ATP use in isolated hearts which have been pre-exposed to agents known to improve postischemic function and blockers/activators of phosphatases, MAPKs, and PKC, (ii) measures of actomyosin ATPase in ventricular myocytes treated as above and also which have different, individual phospho-proteins altered in a way consistent with phosphatase-MAPK activation, and (iii) biochemical assays to monitor phosphatase and MAPK activity under the various conditions. The new information obtained will establish both the second messengers responsible for, and the molecular basis of altered myofilament function which leads to improved postischemic function. The significance of this work is it will suggest novel clinical therapies that may be effective in decreasing ischemic myocardial damage.